Method of development of liquid electrostatic images using an hydrophobic barrier liquid

ABSTRACT

An improved method of developing a latent electrostatic image disposed on a surface comprises depositing a barrier liquid and an aqueous liquid developer on said surface, holding said barrier liquid and developer on said surface until the developer displaces the barrier liquid from the latent electrostatic imagebearing areas of said surface to form a visible image and thereafter fixing the visible image on a receptor sheet, which may be the said surface or one or more separate sheets placed in contact therewith. The barrier liquid is hydrophobic, and the developer carries a coloring agent which can be an organic dye, inorganic pigment, or the like. Fixing can be accomplished merely by drying the coloring agent in place. The surface tension of the developer is lowered sufficiently by the electric field surrounding the latent electrostatic image to effect the desired physical displacement of the barrier liquid.

United States Patent [1 1 Anderson 14 1 May 27, 1975 [75] Inventor:Terry G. Anderson, Pasadena. Calif.

[73] Assignee: Bell & Howell Company, Pasadena,

Calif.

22 Filed: Dec.1l, 1972 211 Appl.No.:314,2l2

[52] US. Cl. 96/1 LY; 117/37 LE; 252/621, 96/76 R [51] Int. Cl G03g13/10 [58] Field of Search 117/37 LE; 96/1 LY, l R, 96/76 R; 252/621 L[56] References Cited UNITED STATES PATENTS 2,558,856 7/1951 Land 96/76R 3,001,888 9/1961 Mctcalfe et al. 252/621 L 3,068,115 12/1962 Gundlach96/1 LY 3,256,197 6/1966 Fanser et 252/621 L 3,262,806 7/1966 Gourge96/1 R X 3,328,193 6/1967 Oliphant ct al, 117/37 LE 3,498,917 3/1970Witter 117/37 LE 3,512,965 5/1970 Matkas 117/37 LE X 3,540,885 11/1970Honjo et al 117/37 LE X 3,560,203 2/1971 Honjo ct al 117/37 LE X3,625,897 12/1971 Machida ct al. 252/621 L 3,642,471 2/1972 Sato et a1117/37 LE X 3,650,738 3/1972 Ikeda et al, 117/37 LE X 3,703,399 ll/1972Tazaka et al. 117/37 LE FOREIGN PATENTS OR APPLICATIONS 729,134 3/1966Canada 117/37 LE 1,223,984 3/1971 United Kingdom 96/1 LY 873,080 7/1961United Kingdom 96/1 LY Primary E.\'aminerRoland E. Martin, Jr.

Attorney, Agent, or Ft'rmNilsson, Robbins, Bissell, Dalgarn & Berliner57] ABSTRACT An improved method of developing a latent electrostaticimage disposed on a surface comprises depositing a barrier liquid and anaqueous liquid developer on said surface, holding said barrier liquidand developer on said surface until the developer displaces the barrierliquid from the latent electrostatic imagebearing areas of said surfaceto form a visible image and thereafter fixing the visible image on areceptor sheet, which may be the-said surface or one or more separatesheets placed in contact therewith. The barrier liquid is hydrophobic,and the developer carries a coloring agent which can be an organic dye,inorganic pigment, or the like. Fixing can be accomplished merely bydrying the coloring agent in place. The surface tension of the developeris lowered sufficiently by the electric field surrounding the latentelectrostatic image to effect the desired physical displacement of thebarrier liquid.

18 Claims, 8 Drawing Figures METHOD OF DEVELOPMENT OF LIQUIDELECTROSTATIC IMAGES USING AN HYDROPI-IOBIC BARRIER LIQUID FIELD OF THEINVENTION The present invention generally relates to image reproductionand more particularly to the conversion of latent electrostatic imagesto visible fixed images.

BACKGROUND AND SUMMARY OF THE INVENTION Most electrostatic copyingmethods which utilize liquid toners require the use of light sensitivepaper, which tends to gray in background areas, show loss of contrastand produce only a single copy from each exposure. Conventionalelectrophoretic development utilizing charged particles in an insulatingmedium results in fixed images with lowered contrast in the backgroundareas due to migration of the toner particles. Other conventionalcopying methods are also subject to such difficulties as lack of highimage density, inability to provide a choice of positive or negativedevelopment. In addition, many conventional copying methods arerelatively extensive and complicated, frequently requiring the use ofskilled personnel to control variables within critical limits. Prior artincludes the following US. Pat. Nos. 3,124,446; 3,245,381; 3,285,741;3,367,791; 3,391,082, 3,425,829; 3,455,240; 3,512,965; 3,551,146;3,556,784; 3,559,570; and 3,560,204.

Accordingly, it would be highly desirable to provide a simple,inexpensive method of electrostatic copying which could be utilized byunskilled personnel to provide one or a plurality of clear, crisp copiesof high contrast on ordinary paper and which would provide a choice ofnegative or positive development.

The present invention provides a method for satisfying the foregoingneeds. A barrier, hydrophobic liquid and an aqueous liquid developer aredeposited sequentially on a charged surface, the developer liquidpreferentially wetting the charged areas of the surface by displacementof the barrier liquid. The barrier liquid protects the uncharged regionsfrom contact with developer liquid providing the background and imagecontrast. The developer liquid carries a coloring agent which can be anorganic dye, inorganic pigment, or the like. Fixing can be accomplishedmerely by drying the coloring agent in place. Alternatively, the imagecan be transferred to one or more separate sheets of'paper. The processcan be easily, rapidly desired. effectively carried out by unskilledpersonnel to produce one or a plurality of clear, sharp copies of anydesired color, even on ordinary paper, if desired. Background areas ofthe copy are of high contrast to the high density images. Positive ornegative development can be provided by simply applying an appropriatepotential across the imaged sheet while applying developer.

Specifically, an aqueous liquid developer is employed, eithersimultaneously, e.g., in emulsion form, with a hydrophobic barrierliquid or separately after application of the barrier liquid to asurface bearing the latent image. The coloring agent in the developerneed not include charged particles, and may be inorganic pigment ororganic dye. The electric field surrounding the electrostaticlatentimage decreases the surface tension of the developer sufficientlyto cause it to displace the barrier liquid from the image areas at thephotoconductive surface. Fixing of the resulting visible image can occurwith or without a prior print-off to one or more sheets of copy paper.Thus, the present method is readily adaptable to rapid multiple copyingfor office or home use.

While the following description of the invention will refer toelectrostatic images formed on a photoconductive surface, the inventionis also applicable to the development of static charge patterns ondielectric papers and the like.

Further features of the present invention are set forth in the followingdetailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A shows a system comprising aphotoconductive surface bearing a latent electrostatic image, thesurface being disposed on a grounded substrate;

FIG. 1B depicts the system of FIG. 1A after application thereto of abarrier liquid;

FIG. 1C depicts the system of FIG. 1B during application of an imagedeveloper by a roller;

FIG. 1D depicts wet visible image resulting from the application asshown in FIG. 1C;

FIG. 1E depicts the visible image of FIG. 1D permanently fixed on areceptor sheet, in this instance the photoconductive surface;

FIG. 2 schematically depicts in side elevation (partly in section) thesystem of FIG. 1 adapted for use in a negative development mode;

FIG. 3 schematically depicts the simultaneous application of an emulsionof developer and barrier liquid to a photoconductive surface from abreakable pod; and

FIG. 4 is a diagrammatic portrayal of the surface tension relationshipbetween developer liquid and barrier liquid.

DETAILED DESCRIPTION FIG. 1A

FIG. 1 schematically depicts a typical system in which the presentmethod is practiced. Thus, as shown in FIG. 1A, a latent electrostaticimage 12 is provided on a photoconductive surface 14 which may comprisea conventional zinc oxide coating disposed on a paper sheet 16 or othersupporting material coated with zinc sulfide, cadmium sulfide, cadmiumselenide, mercuric oxide, or the like. Normally, the selectedphotoconductive material is disposed in fine particulate form in a resinbinder, such as polystyrene, polyvinyl ester or melamine formaldehyde orthe like. The supporting material can be, for example, a cellulosematerial such as paper or the like which has been treated to impartconductivity. A particularly suitable photoconductive surface is zincoxide in a resin binder coated on a conductive paper support.

The support 16 is disposed on an electrically conductive substrate 18which may be a metal plate or foil of aluminum, bronze, copper, lead,zinc, tin, or the like, electrically grounded as shown at 20 in FIG. 1A.

The latent electrostatic image 12 can be provided in any conventionalmanner, as by subjecting the photoconductive surface 14 in theillustrated system to a corona discharge so as to cause a uniformelectrostatic charge to be deposited thereon. The thus-chargedphotoconductive surface 14 can be exposed to light or otherelectromagnetic radiation to which the photoconductive material issensitive. The areas impinged upon by such radiation are dischargedwhile the areas not irradiated retain their charge. accordingly, alatent electrostatic image 12 shown in FIG. IA is provided. This imageis then developed by the present method.

In accordance with the present method a barrier liquid and an aqueousliquid developer bearing a coloring agent are deposited on thephotoconductive surface 14 by any suitable means, either simultaneouslyor by application of the barrier liquid first and the developerthereafter. Thus, the barrier liquid and developer may be in emulsionform, if desired, for simultaneous application.

As barrier liquid one can use any suitable volatile, insulating,hydrophobic liquid immiscible with the developer and capable of wettingthe photoconductive surface 14. In this regard, suitable nonpolarorganic liquids, such as hydrocarbons, preferably paraffinic, orpycloparaffinic hydrocarbons, may be employed as the barrier liquid.Selection of the particular barrier liquid to liquid employed willdepend on the particular tech- 1lqUC used in the present method,including the particular photoconductive surface, the particular aqueousdeveloper and its ingredients, and the like. Suitable barrier liquidsinclude iso-paraffin, specifically the mixture sold under the trademarkIsopar-G, by Humble Oil and Refining Company, as well as paraffin oil,nliexane, n-heptane, cyclohexane, benzene, toluene, xylene, carbontetrachloride, perchloroethylene, trichlorotrifluoroethane, and the likehydrocarbons, substituted or unsubstituted and aromatic, aliphatic orcyclic aliphatic, mixtures thereof and the like.

The barrier liquid performs the function of blocking the developer fromaccess to the photoconductive sur face 14 except in those areas bearingthe electrostatic image 12. In those areas, the electrical field causesthe surface tension of the developer to decrease to such an extent thatthe developer in those areas physically displaces the barrier liquid,preferentially wets the photoconductive surface 14 and, accordingly,forms the desired visible image corresponding to the latentelectrostatic image. In the remaining areas, the barrier liquidcontinues to prevent the developer from wetting the photoconductivesurface 14 and laying down a visible pattern. The line of demarcationbetween the developer-occupied areas and the liquid barrier-occupiedareas lS sharp, so that the visible image exactly reproduces the latentimage and so that strong contrast exists between the visible image andthe image-free background. and,

The developer is aqueous and includes a suitable col ormg agent whichcan be organic dye, inorganic pigment or a mixture thereof. For example,pigment base, such as black poster paint, designers gouache, acrylicemulsions, carbon black latexes, iron oxide, cadmium yellow, cadmium redand the like can be used. Other examples of inorganic coloring agentsinclude: potassium permanganate, ferric chloride, cobaltous chloride,cadmium sulfide, cadmium selenide, ceric nitrate ind cupric oxide.

Organic pigments and dyes such as Methylene Blue i available fromEastman Kodak Co.), Brilliant Yellow [available from Harleco ChemicalCo. Crystal Violet, Malachite Green, Victoria Blue, Carmine Red, NigroimC, Chloramine Black, Razon Black, Methylene Viplet, Rose Bengal andQuinoline Yellow (available Trom Allied Chemical Co.) can be used. Otherexamples of suitable organic coloring agents include Eriochrome Black T,Alizarin and Indigo Carmine.

The coloring agent can be used with aqueous developer in any suitableconcentration, for example, about 310 percent, by weight of developer.If the coloring agent is insoluble or' has low solubility in water, amutual solvent such as dimethyl formamide can be used, for example, in aconcentration of about 1-1 5 percent, by weight of the developer.Alcohols, such as isopropyl alcohol, methanol, ethanol and the like canbe used for similar purposes. Moreover, such additives are surfacetensionlowering agents and can be advantageously used to adjust theinitial surface tension of the aqueous developer to a value above butclose to that required for displacement of the barrier liquid, Othersurface tension-lowering agents miscible with water include acetone.methyl ethyl ketone, lauryl sulfonate and dodecyl sulfonate. Such agentscan be used in any suitable concentration, for example, about 15 percentby weight of the developer.

Normally, water is present in the developer in a major proportion, forexample, about -90 percent by weight of the developer. When inorganicpigments are used, suspension agents may be employed in order to keepthe pigment particles (usually of very small size) in suspension. Forexample, carboxymethyl cellulose or commercial dispersants such as soldunder the name of DAXAD (W. R. Grace & Co.), Dispex (Allied Colloids) orTAMOL (Rohm & Haas), can be used, in concentration of, for example,about l to about 10 percent by weight of the developer.

It will be understood that the coloring agent of the developer isselected such that it is insoluble or has a low solubility in theparticular barrier liquid under the operating conditions, in order toavoid bleeding" of coloring into barrier liquid and staining of thebackground areas surrounding the visible image pattern. Moreover, itwill be understood that the developer is aqueous and volatile, exceptfor the coloring agent and the suspension agent (if used) and does notdepend on charged particles to deposit the coloring agent in place onthe latent image areas 12 of photoconductive surface 14.

Fixing of the coloring agent to render permanent the visible imageproduced thereby requires only drying of the developer, which can beaccelerated by heat, air circulation and the like. Before such drying,the visible image can be easily printed off on plain ordinary copymaterial such as bond paper.

In the event that the developer and barrier liquid are to be appliedsimultaneously as an emulsion, an emulsifying agent can, if desired, beused, for example, calcium or magnesium soaps (which promote dispersionof aqueous liquids), in a concentration of, for example, about 0.01 toabout 1 percent by weight of the developer, in order to aid theformation of the emulsion. Normally, in such emulsions, the barrierliquid and developer are in a volume ratio of between about 3:1 andabout 1:3, although the particular ratio is not critical.

Operation In FIG. 1B, the system of FIG. 1A is shown, but after a layer22 of barrier liquid has been applied to the entire photoconductivesurface 14. The barrier layer 22 can be applied by any suitable means,as by a steel roller (not shown), after which (according to the embodiment of the present method shown in FIG. 1), and referring to FIG. 1C, aroller 24 or the like uniformly applies a coating 26 of aqueousdeveloper over the barrier liquid layer 22.

As previously described, the electrical field sur rounding the image 12reduces the surface tension of coating 26 in the areas of image 12 tosuch an extent that coating 26 displaces layer 22 in those areas to forma wet visible image, on photoconductive surface M, as shown in FIG. 1D.This image can then be fixed in place by drying or the like, withconsequent removal of the remainder of coating 26 and layer 22, toprovide the result shown in FIG. 1E, a permanent visible image exactlyreproducing the original latent image.

Reversal Should it be desired to provide a visible image which is thenegative or reverse of that provided by the procedure depicted in FIG.1, this can easily be accomplished by utilizing a system such as isshown schematically in FIG. 2. Thus, in FIG. 2 components similar tothose shown in FIG. I bear the same numerals but are succeeded by theletter a. In FIG. 2, a latent image is depicted at 12a disposed on aphotoconductive surface 140 on a support 16a in turn backed by aconductive substrate 180 electrically grounded at 20a. Substrate 18a isalso electrically interconnected with a metal roller 24a and a powersource 28.

A layer 22a of barrier liquid is disposed on the photoconductive surface14a with a coating 26a of a developer overlying layer 22a. Due to theapplication of a bias potential from power source 28, the electric fieldin the areas of the image 12a is neutralized and an electric field isprovided on the image-free areas. The strength of the charge depends onthe charge density of the latent image, e.g., a source of about 400volts is generally sufficient for images obtained with commercial coronadischarge units. This results in deposition of coating 26a only in theimage-free areas, due to a decrease of surface tension in the developerin those areas. The system of FIG. 2 thus provides a fixed visible imagewhich is a negative or reversed image of the original latentelectrostatic image, all by the mere activation of power source 28. Thisis a substantial advantage over many conventional copying systems.

Emulsion The embodiments of the present method illustrated in FIGS. 1and 2 have been described primarily in terms of sequential applicationof the barrier liquid and developer. However, as previously noted, theseliquids can be applied simultaneously, if desired, for example, in anemulsion, through the use of means such as are depicted schematically inFIG. 3. Thus, in FIG. 3, a photoconductive sheet 30 passes between apair of pinch rollers 32. An absorbent web 34 bearing a pod 36 on thesurface thereof facing sheet 30 also passes be tween rollers 32 alongwith sheet 30. Rollers 32 crush the pod 36 so as to release an emulsionof developer and barrier liquid therefrom, which emulsion is uniformlydistributed on sheet 30 by rollers 32 for development of a visible imagein accordance with the present method. Such pod could, if desired,contain a mixture of the developer and barrier liquid or either of theseliquids by itself. A plurality of such pods 36 can then be spaced alongweb 34 for sequential release of liquid at rollers 32. In place of thepod 36 or plurality of pods one can use microencapsulated liquids.

Copying and Fixing After the visible image is formed, as describedabove, the wet image on the photoconductive surface can either be fixedin place, as by drying or the like, or printed off on one or a pluralityof receptor sheets, such as plain bond paper or the like, a distinctadvantage over many conventional systems. If the visible image is to befixed on the photoconductive surface, that surface functions as thereceptor sheet. Once the coloring agent is depleted during multiplecopying, reapplication of the developer (and barrier liquid as needed)to the photoconductive surface results in a new visible wet image fromwhich to reproduce further copies of the original latent image. Thus,reexposure of the photoconductive surface is not required in order toproduce a plurality of copies. A saving in time and effort is thuseffected, with a reduction in processing cost.

It will be understood that the fixing of the visible image to apermanent form can be accomplished by merely volatilizing the volatileconstituents of the developer, leaving the non-volatile coloring as thevisible image. Heating, air circulation and other ways of controllingthe volatilization can be employed, as previously noted. Other suitablemeans can be used, such as chemical reactions, including complexformation and polymerization, depending on the particular coloring agentand materials deposited therewith.

General Although I do not intend to rely upon any particular theory orprinciple of operation, the relationship of the developer liquid A tothe immiscible barrier liquid B on the photoconductive surface S can bedepicted as illustrated in FIG. 4. In order for liquid A to displaceliquid B from the surface S, the angle 9, must be acute [Adam, N.K. ThePhysics and Chemistry of Surfaces, Third Edition, Oxford ChemistryPress, London (1941), page I88]. The angle 0 will be acute only ifz 7,,cos 0,, 2 7 cos 6,,

where 'y,, and y are surface tensions of liquids A and B with respect toair and 6,, and 0 are contact angles of each of the two liquids on thesurface S in the absence of the second liquid. In the present case, thebarrier liquid wets the surface S (0 0, y z 20 dy nes/cm, as anexample). With water as the liquid phase A, 0 80 and y,, z dynes/cm, andthe condition of formula (1) is not fulfilled. An electric field acts todecrease 7 and 6,,. At a critical field strength these changes cause theconditions of formula l to be satistied, and liquid A displaces liquid Bfrom the surface in regions experiencing the electric field. Where theelectric field is not sufficiently strong, however, the barrier liquid 8remains, forming a hydrophobic surface and preventing discoloration ofthe background.

The following examples will further illustrate the invention.

EXAMPLE I A latent electrostatic image can be formed on the surface ofZinc oxide paper (zinc oxide-resin binder coating) disposed on agrounded metallic plate by exposing that surface to a corona dischargeand then exposing the resulting charged surface to a light pattern TABLEI lngredicnts Concentration (7( by weight) water 45 Ecco CoatSec(coloring 45 agent) lsopropyl Alcohol (surface tension-loweringagent) 10 A conductive black latex from Emerson 8L Cumings' The latentelectrostatic image develops to a visible image when the roller is movedat about 2 inches per second, after which the image zinc oxide paper isheated to about 90C for about seconds until the visible image dries to apermanent state and the remainder of the developer evaporates, alongwith the barrier liquid. The finished image paper is then removed. Theimage is sharp, of high contrast with an unblemished background and offine quality.

In a first parallel run, before drying the visible image, it can beprinted off by contact with a sheet of plain bond paper.

ln a second parallel run the barrier liquid and developer can be appliedsimultaneously as an emulsion. The emulsion is formed by mixing thedeveloper and barrier liquid together.

EXAMPLE 2 The procedures set forth for the first run and the twoparallel runs in Example 1 are followed, except that a barrier liquidcomprising lsopar G and a developer with ingredients as set forth inTable 11 below are substituted for those called for in Example 1, toyield comparable results.

TABLE ll Ingredients Concentration (71 by weight) water 85 Capri Bluedye (coloring agent) 5 lsopropyl Alcohol (sur face tension-loweringagent) out in office-type or home use equipment of simple constructionand high durability. Other advantages of the present method are as setforth in the foregoing. Various modifications, alterations, changes andadditions can be made in the present method, its steps and parameters.All such modifications, alterations, changes and additions as are withinthe scope of the appended claims form part of the present invention.

I claim:

1. A method of development of electrostatic images, which methodcomprises:

depositing a layer of hydrophobic barrier liquid on a surface bearing alatent electrostatic image, said barrier liquid wetting said surface;

depositing a layer of aqueous developer liquid, containing a majorproportion by weight of water and a minor proportion by weight of asurface tensionlowering agent, on said layer of barrier liquid, saidliquid developer being immiscible with said barrier liquid and bearing acoloring agent dissolved or dispersed therein having no or lowsolubility in said barrier liquid;

holding said barrier liquid and developer liquid on said surface untilsaid developer liquid displaces said barrier liquid from the areas ofsaid surface bearing said image and wets said surface areas so as toform a visible image; and

thereafter fixing said visible image on a receptor sheet.

2. The method of claim 1 wherein said liquid developer has an initialsurface tension higher than that of said barrier liquid and wherein saidsurface tension of said developer is lowered sufficiently by theelectric field accompanying said latent electrostatic image to enablesaid developer to displace said barrier liquid in said areas.

3. The method of claim 2 wherein said coloring agent of said developeris devoid of electrically charged particles.

4. The method of claim 3 wherein said coloring dye is selected from thegroup consisting of inorganic pigment and organic dye and mixturesthereof.

5. The method of claim 2 wherein said barrier liquid comprises non-polarorganic liquid.

6. The method of claim 5 wherein said non-polar organic liquid comprisesa hydrocarbon.

7. The method of claim 6 wherein said hydrocarbon comprises a liquidparaffinic hydrocarbon.

8. The method of claim 1 wherein said coloring agent comprises anorganic dye and wherein said developer also includes a minor proportionof a solvent for said dye.

9. The method of claim 8 wherein said developer comprises at least about60 percent by weight of water, about 10-15 percent by weight of surfacetensionlowering agent, up to about 15 percent by weight of dyestuffsolvent, and about 3-10 percent by weight of dye stock.

10. The method of claim 9 wherein said surface tension-lowering agentcomprises isopropyl alcohol, said dyestuff solvent comprisesdimethylformamide and said dye comprises chlorophyll.

11. The method of claim 10 wherein said developer is deposited on saidsurface by a roller.

12. The method of claim 11 wherein said roller is electricallyconductive.

13. The method of claim 12 wherein said surface is photoconductive anddisposed on an electrically grounded metallic substrate and wherein saidroller is electrically connected to a power supply of polarity to inducenegative development of said images.

14. The method of claim 1 wherein said visible image is fixed byallowing it to dry on said surface, and wherein said surface is saidreceptor sheet.

15. The method of claim 1 wherein said visible image is transferred fromsaid surface to a separate receptor sheet by contact therewith and isthen fixed on said separate sheet.

(576 UNiTED STATES PATENT OFFICE CERTIFIQATE 0F CGRREfiTlGN Patent No.3! 5 9 0 Dated May 27, 1975 Q 'yj q fl Terrv G. Anderson It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

a Column 1, line 51, change "desired" to and--.

Column 3, line 2, change "accordingly" to -Accordinqly.

line 50, delete "and,"

Sugncd and Sealed thls twenty-sixth Day Of August 1975 [SEAL] Q Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer (ummissimwr oj'Parenrsand Trademarks

1. A METHOD OF DEVELOPMENT OF ELECTROSTATIC IMAGES, WHICH METHODCOMPRISES: DEPOSITING A LAYER OF HYDROPHOBIC BARRIER LIQUID ON A SURFACEBEARING A LATENT ELECTROSTATIC IMAGE, SAID BARRIER LIQUID WETTING SAIDSURFACE; DEPOSITING A LAYER OF AQUEOUS DEVELOPER LIQUID, CONTAINING AMAJOR PROPORTION BY WEIGHT OF WATER AND A MINOR PROPORTION BY WEIGHT OFA SURFACE TENSION-LOWERING AGENT, ON SAID LAYER OF BARRIER LIQUID, SAIDLIQUID DEVELOPER BEING IMMISCIBLE WITH SAID BARRIER LIQUID ANB BEARING ACOLORING AGENT DISSOLVED OR DISPERSED THEREIN HAVING NO OR LOWSOLUBILITY IN SAID BARRIER LIQUID; HOLDING SAID BARRIER LIQUID ANDDEVELOPER LIQUID ON SAID SURFACE UNTIL SAID DEVELOPER LIQUID DISPLACESSAID BARRIER LIQUID FROM THE AREAS OF SAID SURFACE BEARING SAID IMAGE;AND WETS SAID SURFACE AREAS SO AS TO FORM A VISIBLE IMAGE; ANDTHEREAFTER FIXING SAID VISIBLE IMAGE ON A RECEPTOR SHEET.
 2. The methodof claim 1 wherein said liquid developer has an initial surface tensionhigher than that of said barrier liquid and wherein said surface tensionof said developer is lowered sufficiently by the electric fieldaccompanying said latent electrostatic image to enable said developer todisplace said barrier liquid in said areas.
 3. The method of claim 2wherein said coloring agent of said developer is devoid of electricallycharged particles.
 4. The method of claim 3 wherein said coloring dye isselected from the group consisting of inorganic pigment and organic dyeand mixtures thereof.
 5. The method of claim 2 wherein said barrierliquid comprises non-polar organic liquid.
 6. The method of claim 5wherein said non-polar organic liquid comprises a hydrocarbon.
 7. Themethod of claim 6 wherein said hydrocarbon comprises a liquid paraffinichydrocarbon.
 8. The method of claim 1 wherein said coloring agentcomprises an organic dye and wherein said developer also includes aminor proportion of a solvent for said dye.
 9. The method of claim 8wherein said developer comprises at least about 60 percent by weight ofwater, about 10-15 percent by weight of surface tension-lowering agent,up to about 15 percent by weight of dyestuff solvent, and about 3-10percent by weight of dye stock.
 10. The method of claim 9 wherein saidsurface tension-lowering agent comprises isopropyl alcohol, saiddyestuff solvent comprises dimethylformamide and said dye compriseschlorophyll.
 11. The method of claim 10 wherein said developer isdeposited on said surface by a roller.
 12. The method of claim 11wherein said roller is electrically conductive.
 13. The method of claim12 wherein said surface is photoconductive and disposed on anelectrically grounded metallic substrate and wherein said roller iselectrically connected to a power supply of polarity to induce negativedevelopment of said images.
 14. The method of claim 1 wherein saidvisible image is fixed by allowing it to dry on said surface, andwherein said surface is said receptor sheet.
 15. The method of claim 1wherein said visible image is transferred from said surface to aseparate receptor sheet by contact therewith and is then fixed on saidseparate sheet.
 16. The method of claim 15 wherein said visible image isprinted off on a plurality of separate receptor sheets withoutredevelopment and said printed off images are fixed by drying on saidreceptor sheets.
 17. The method of claim 15 wherein said surface isphotoconductive zinc oxide paper and said separate receptor sheet isordinary paper.
 18. The method of claim 1 wherein at least one of saiddeveloper and barrier liquid is deposited on said surface by releasefrom a pod during crushing thereof by a roller.